1. Field of the Invention
The present invention concerns data communications.
It is more particularly concerned with data communication systems including a "normal" link for transmitting data between two terminal equipments, on which the data is normally transmitted, and a "standby" link used if the normal link fails.
2. Description of the Prior Art
The normal link is usually a leased line and the standby link is provided by the public switched telephone network. In each case, both these terms are used hereinafter without distinction.
There are various known methods for switching from the normal link to the standby link after malfunctioning of the normal link is detected.
The present invention is concerned with returning to the normal link after using the standby link.
Known methods for returning to the normal link involve, for example, regularly attempting transmission on the normal link, stopping transmission on the standby link at these times, or continuing transmission on the standby link while simultaneously verifying the continuity of the normal link using a specific signalling frequency transmitted over the latter.
The drawback of these methods is that they either disrupt the transmission of data at the tinge or this operation or that they do not verify the continuity of the normal link under its actual operating conditions. A further drawback is that they authorize the return to the normal link when the transmission quality on the normal link has not necessarily returned to normal.
An object of the present invention is a method whereby these drawbacks can be avoided.